CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority of Japanese Patent Application No. 10-111401, filed on Apr. 8, 1998, the contents of which are hereby incorporated herein by reference.
1. Field of the Invention
The invention concerns apparatus in which CVD (chemical vapor deposition) or etching, for example, is executed using plasma, and in particular it concerns plasma processing apparatus which is characterized by the construction of the transmission path by which the high frequency power is supplied to the plasma.
2. Discussion of Related Art
Surface processing such as CVD and etching is carried out widely using plasma at the present time, and LSI (large scale integrated circuits) and FPD (flat panel devices) are being manufactured using such techniques. Various systems are known for generating the plasma, but the plasma generating systems in which a high frequency discharge is used are widely employed because they enable a stable plasma to be obtained over a wide area. Plasma generating systems in which a high frequency discharge is used can be broadly classified into the capacitively coupled type systems and the inductively coupled type systems, and the invention of this present application concerns primarily plasma generation systems of the capacitively coupled type.
Plasma generating apparatus of the capacitively coupled type which is suitable for use with high frequencies (in the VHF/UHF band) has been disclosed in the specification of U.S. Pat. No. 5,210,466. A front cross sectional drawing of such conventional plasma processing apparatus is shown in FIG. 7 of the present application. The vacuum chamber 10 is constructed from a cylindrical side wall 12, a top-plate 14 and the bottom-plate 16. The cylindrical cathode 18 is present in the middle of the interior of the vacuum chamber 10, and is surrounded via the insulator 20 by the annular conductor 22. Thus, the transmission path inside the chamber comprises the cathode 18, the insulator 20 and the annular conductor 22. The vacuum chamber 10 is pumped out with the pumping apparatus 34. The discharge gas is delivered into the vacuum chamber 10 at an appropriate mass flow rate through the gas distribution plate 32 of the gas delivery apparatus and maintained at the prescribed pressure. High frequency energy from the high frequency power supply 26 is conducted into the abovementioned transmission path 24 within the vacuum chamber via the matching circuit 28 and plasma is generated between the cathode 18 and the anode (principally the gas distribution plate 32). Thus, in an etching process, for example, the etching is achieved by the action of the ions in the plasma on the substrate 36 on the cathode 18.
By adopting a transmission path of the type described above, the plasma processing apparatus shown in FIG. 7 enables a plasma to be generated at high frequencies of from 50 to 800 MHZ. The electrode sheath voltage is reduced by using such a high frequency and electrical damage to the substrate is minimized, and deposition rates or etch rates which are adequate for industrial purposes can be attained.
In the conventional apparatus shown in FIG. 7, the transmission path 24 within the chamber is a single coaxial line, and this coaxial line has been designed along the following lines. If the characteristic impedance of the coaxial line is Z.sub.0, its length is L and the phase constant is .beta., then the input impedance Z.sub.in as seen from the matching circuit 28 of a coaxial line with a plasma providing a load impedance (Z.sub.L) is given by the following equation (1): EQU Z.sub.in =Z.sub.0 [Z.sub.L +jZ.sub.0 tan(.beta.L)]/[Z.sub.0 +jZ.sub.L tan(.beta.L)] (1)
It is clear from this equation that when Z.sub.0 =Z.sub.L, then Z.sub.in =Z.sub.0 irrespective of L. That is to say, by establishing a coaxial line which has a characteristic impedance Z.sub.0 which is equal to the plasma impedance Z.sub.L, the impedance of the coaxial line as seen by the matching circuit 28 has a constant value (equal to the plasma impedance), and the matching conditions are ideal. However, in practice the plasma impedance varies depending on the state of the plasma and it has a certain width, and so the characteristic impedance Z.sub.0 of the coaxial line is set roughly to the mid-value of the width of this variation. In this case, rigorously, Z.sub.0 is not equal to Z.sub.L and the line length L has an effect, and the matching conditions of the matching circuit become poor. It is necessary to make the line length L much shorter than a quarter wavelength to minimize this extent of this effect.
In the conventional apparatus shown in FIG. 7 of the '466 patent, the transmission path 24 within the chamber is a coaxial line with the cathode 18 as the internal conductor and the annular conductor 22 as the external conductor. The substrate 30 must be located on the cathode 18 and so the diameter of the cathode 18 must always be larger than the diameter of the substrate. The annular conductor 22 is on the outside of the cathode 18 and the outer wall 12 is on the outside of this. With apparatus which has such a construction the external dimensions of the outer wall 12 of the vacuum chamber become very large as the substrates become larger, and the weight of the apparatus is increased. As a result of the latest technical developments, a need for the size of the wafers which are being used for LSI to be increased to a diameter of 300 mm has arisen. Furthermore, in the case of FPD there is a need for an increase in the size of the glass substrates to 550.times.650 mm or to 1 meter square. Hence, the diameter of the cathode 18 has to be increased and, as a result, the weight of the apparatus is increased and there is a further problem in that the manufacturing costs are also increased.